1. Field of Invention
The invention relates to an automatic detecting device for signals of land pre-pits on digital video/audio optical disks, utilizing gain adjustment of push-pull signals to raise the signal to noise ratio of the land pre-pits signal to the wobble signal. Digital logic computation is also utilized to recognize the practical position of the land pre-pits. The level automatic detection of the RC circuit and the fixed slice level compose an automatic slice level, which is the basis for detecting the land pre-pits.
2. Related Art
A current write-once DVD (DVD-R) or re-writable DVD (DVD-RW) has the same structure, where records relative data, such as laser wavelength, write power, write strategy, manufactures, or track positions. FIG. 1A illustrates the schematic diagram of the data tracks of a DVD. The land pre-pits 15 of a DVD-R or a DVD-RW are formed in the grooves 17 relative to the peak of the wobbles. The grooves 11 and the lands 17 interlace with each other. After data are recorded in the grooves 11 of the disk, a mark 12 is written in the pre-pits area 15 of the land 17, as shown in FIG. 1B. The un-continuity of the lands 17 in each land pre-pits area 15 results in edge effect of heat diffusion. The mark 12 in the grooves 11 diffuses to the pre-pits area 15. The LPP signal is thus influenced. The amplitude of LPP signals relative to the wobble signal decreases after data are written onto the disk. This leads to decrease of the signal to noise ratio.
FIG. 2A shows the waveform of land pre-pits (LPP) signals not being affected by written data before writing a DVD. A constant LPP slice level, as denoted by dot lines in the figure, is adopted to obtain correct digital LPP signals through comparators. FIG. 2B shows the waveforms after the DVD is written. The LPP signal is usually generated by push-pull approach and constant LPP slice level. The LPP signal is illustrated as waveform A. After data are written on the disk, the amplitude of the LPP signals relative to the wobble signals recesses, which is illustrated as waveform B in the figure. In this situation, adoption of the constant LPP signal slice level brings low accuracy of the LPP signal. The accuracy is still low, even after being protected through Error Correcting Code (ECC). The phenomenon affects access of the lead-in information of the disk drive and access of the address during track jump. The characteristic of the LPP signal of the inner circle and that of the outer circle is not in unanimity during tracking following on the same disk. Furthermore, the influence of written data, the difference between different disks, and writing times lead to accuracy decrease of the LPP signals. Therefore, an automatic level adjusting mechanism is necessary to obtain correct LPP signals under all circumstances.
FIG. 3 describes the circuit for generating LPP signals of the prior art. The circuit involves comparing the fixed LPP slice level V1 and the LPP signal V2 in the comparator 31 to obtain the LPP slice signal V12. The LPP slice signal V12 is synchronized with the wobble clock V3 through the noise gate 33 in the circuit to eliminate unnecessary glitches. The filtered LPP slice signal V4 is delivered to a LPP decoder 35. Under the influence of high frequency RF signals, the amplitude of LPP signals relative to the wobble signal decrease after data are written onto the disk. Therefore, the credibility of the LPP slice signal before writing data onto the DVD is better than that after writing data onto the DVD.
FIG. 4 illustrates the block diagram of the circuit of the prior art for amending the slice level of land pre-pits. The circuit improves the method of the constant slice level of land pre-pits. The land pre-pits signal V2 is delivered to a level limiter 41 and filtered by a band pass filter 43, thereby amending the slice level. The filtered signal is then computed with the constant slice level V1 and the computed signal is then sent to the comparator 31 to compare with the land pre-pits signal V2. The land pre-pits signal V12 is thus obtained. In order to eliminate the unnecessary pulse interference, the land pre-pits signal V12 is synchronized with the wobble clock V3 through the noise gate circuit 33. The protected land pre-pits signal V4 is then generated and delivered to the land pre-pits decoding circuit 35. However, the affection of the adjacent land pre-pits causes the insufficient amendment in the slice level and erroneous judgment of land pre-pits. The accuracy rate of the LPP signals is not improved too much.
The invention employs automatic detection to detect the variation of the wobble signals to overcome the technical difficulties of prior art, and improves the accuracy rate of the LPP signals in coordination with digital logic computation.